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Re: Is a SERIAL column a "black box", or not?

From: Mark Dilger <pgsql(at)markdilger(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: Is a SERIAL column a "black box", or not?
Date: 2006-04-30 16:14:53
Message-ID: 4454E27D.2090503@markdilger.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> 1. A serial column is a "black box" that you're not supposed to muck with
> the innards of.  This philosophy leads to the proposal that we disallow
> modifying the column default expression of a serial column, and will
> ultimately lead to thoughts like trying to hide the associated sequence
> from direct access at all.

It would be madness to prevent people from accessing the associated sequence.
Assume the following schema:

   CREATE TABLE a (a_id SERIAL NOT NULL UNIQUE, ...);
   CREATE TABLE b (a_fk INTEGER REFERENCES a(a_id), ...);

Now, if I need to insert into both tables a and b, how do I do it?  After
inserting into table a, if I can't access the sequence to get currval, I'll need
to do a select against the table to find the row that I just inserted (which
could be slow), and if the columns other than a_id do not uniquely identify a
single row, then I can't do this at all.

mark

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